US 8068066 B2
An X-band, crossed dipole turnstile antenna configured to be omni-directional with horizontal polarization is disclosed. It comprises a set of two dipole antennas aligned at right angles to each other attached to a common 50 ohm coaxial feedpoint and fed 90 degrees out-of-phase. The antenna pattern is nearly omnidirectional in the horizontal plane. The antenna can be used generally in microwave communications including Digital Radio Frequency Tags (DRaFTs) communicating with airborne and satellite platforms.
1. A microwave turnstile antenna comprising:
a ground plane; and
a pair of crossed dipole wire elements having a spacing from said ground plane and said elements fed 90 degrees out of phase; and
a length of U-shaped coaxial cable in electrical connection between dipole wire elements of said pair of crossed dipole wire elements.
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16. A horizontally polarized X-band turnstile antenna comprising:
a 1.4 inch diameter copper ground plane proximate a skirt;
a pair of crossed dipole elements 0.6375 inch long having a spacing 1.155 inches from said ground plane opposite said skirt, said dipole elements having 90 degree phasing; and
a 0.66 inch long segment of U-shaped coaxial cable in electrical connection between said dipole elements.
17. A microwave frequency tag comprising: an antenna comprising:
a ground plane;
a pair of crossed wire element dipoles spaced from said ground plane and having 90 degree phasing;
a segment of U-shaped coaxial cable in electrical connection between wire dipole elements of said crossed wire element dipoles; and
circuitry in electrical communication with said antenna wherein said microwave frequency tag communicates with a transceiver.
18. The microwave frequency tag of
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20. The microwave frequency tag of
Portions of the present invention were made in conjunction with Government funding under contract number W15P7T-06-C-P422 giving certain rights to the Government.
This invention relates to microwave antennas and, more particularly, to the utilization of a crossed dipole turnstile antenna configured to be omni-directional with horizontal polarization.
Radio frequency communication with air and space platforms provides the opportunity to remotely track objects over large distances. Military operations especially have a need for tracking technology for air-to-ground Combat Identification (CID). This generally includes microwave communications. As an example, a Digital Radio Frequency Tag (DRaFT) can provide flexible, low cost technology to allow radars such as Moving Target Indicator (MTI) and Synthetic Aperture Radar (SAR) to receive data from ground devices. These small, lightweight and affordable RF Tags provide for data extraction from unattended ground sensors and communication with vehicles and personnel throughout an area. This is particularly useful for the identification and location of combined units. Other advanced tag functions include additional communications capabilities for enhanced interoperability with identification and communications systems. These can give the tags dual-mode capability to function as a tag when radar is present or as a more conventional radio beacon device when radar is not available. Another application includes dual-mode tags communicating with Satellite Communication (SATCOM) platforms. Additionally, small-scale tag variations may support other target tracking, substantially enhancing situational awareness and asset identification for ground operations. Tag antenna characteristics include horizontal polarization required to communicate with airborne radar platforms having horizontal (azimuth) polarization. Linear and circular polarization can be employed. Antennas presently used for DRaFTs are very large, waveguide slot antennas. They are typically 7 inches long, 1 inch wide and 0.5 inch deep. What is needed, therefore, are small, inexpensive antennas with horizontal polarization and an omni-directional pattern.
The above problems of waveguide slot antennas are solved by providing a crossed dipole, turnstile antenna over a ground plane. Advantages of the new antenna are that it is small, very inexpensive, omni-directional, and can be built using microwave integrated circuit assembly tools.
The antenna is capable of communicating with loitering platforms, has linear horizontal polarization and is able to handle up to 2 watts continuous wave (CW) power over the frequency of interest. Bi-directional communication is supported with a radiation pattern having transmit/receive reciprocity. It is omnidirectional in azimuth, with wobble less than or equal to 1 dB and an elevation gain of +3 dBi at 45 degrees of elevation. It has small size and light weight.
The invention can be applied to Digital Radio Frequency Tags (DRaFT). It can also be used in other microwave communication systems including but not limited to radios and direction finding equipment.
Embodiments of the invention include a horizontally polarized microwave turnstile antenna comprising a ground plane and a pair of crossed dipole elements having a spacing from the ground plane and the elements fed 90 degrees out of phase. The antenna radiation polarization can be horizontal and the antenna can provide transmit and receive reciprocity. The radiation pattern can be substantially omnidirectional in the plane of the ground plane. The radiation pattern can be circularly polarized. In embodiments, the antenna radiation frequency is in the X-band. The antenna resonant frequency can be 9.5 GHz to 9.8 GHz. For embodiments, the spacing from the ground plane is one-half wavelength. This spacing from the ground plane can be 0.611 inch. For embodiments, the length of the crossed dipole elements is one-half wavelength. For certain embodiments, the length of the crossed dipole elements is 0.7 inch and the ground plane is a copper disk. In another embodiment, the ground plane is proximate a skirt. In yet other embodiments, the ground plane diameter is 1.4 inches. For embodiments, the length of the U-shaped piece of coaxial cable between the two dipoles is selected to produce circularly polarized (CP) radiation.
Yet further embodiments include a horizontally polarized X-band turnstile antenna comprising a 1.4 inch diameter copper ground plane proximate a skirt, a pair of crossed dipole elements 0.6375 inch long having a spacing 1.155 inches from the ground plane opposite the skirt, the dipole elements having 90 degree phasing, and a 0.66 inch long segment of U-shaped coaxial cable in electrical connection between the dipole elements.
Other embodiments include a microwave frequency tag comprising an antenna comprising a ground plane, a pair of crossed dipoles spaced from the ground plane and having 90 degree phasing, and circuitry in electrical communication with the antenna wherein the microwave frequency tag communicates with a transceiver. For embodiments, the microwave frequency tag is associated with personnel or vehicles. In yet other embodiments, the microwave frequency tag is a digital radio frequency tag (DRaFT).
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.
A turnstile antenna is a set of two dipole antennas aligned at right angles to each other attached to a common 50 ohm coaxial feedpoint and fed 90 degrees out-of-phase. The name reflects that the antenna looks like a turnstile when mounted horizontally. When mounted horizontally, the antenna is nearly omnidirectional on the horizontal plane. When mounted vertically, the antenna is directional to a right angle to its plane. In embodiments of the present application, the antenna can be used generally for microwave communications. In particular embodiments, the antenna can be mounted on a vehicle or personnel-carried tag and communicate with a horizontally polarized antenna on an aircraft.
In embodiments, tiny semirigid coaxial cable was used to create the feed and 90 degree phasing. This was at a high frequency (near 10 GHz). The groundplane spacing is important at X-band (and microwave frequencies in general) as are the dipole elements themselves.
Embodiments of the antenna work cooperatively with loitering airborne platforms. Aircraft are typically within 135 nautical miles, line of sight (L.O.S.). The resonant frequency range is 9.5 to 9.8 GHz with linear horizontal polarization and an impedance of 50 ohms. Other attributes include a voltage standing wave ratio (VSWR) less than 1.5:1, a return loss of less than 14 dB, and the ability to handle up to 2 watts (+33 dBm) CW. The radiation pattern has transmit/receive reciprocity supporting bidirectional communication and is omni directional in azimuth with wobble less than 1 dB. Elevation gain is +3 dBi at 45° elevation and radiation efficiency is 92%, with total efficiency of 80%. In embodiments, the ground plane spacing is approximately 0.600 inch. An exemplary connector is a SubMiniature version A (SMA) type. Size and weight are preferably less than 0.5 cubic inch and 1 ounce, respectively.
Antenna embodiments include a manufactured device, a computer simulation of the electrical characteristics of the antenna, and two computer models employing the physical attributes of the turnstile antenna.
The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.